This invention relates generally to powder coatings and, more particularly, to the continuous processing of powder coatings.
Because of increased environmental concerns, much effort has been directed to the problem of reducing pollution caused by the evaporation of solvents from paints. These efforts have led to the development of new coating technologies which eliminate or at least diminish the emission of organic solvent vapors into the atmosphere. Since the mid-1950's, the powder coating technology has been one of the most successful developments in terms of reducing or eliminating solvent vapor emissions.
The use of powder coating compositions can be extremely desirable as such compositions are essentially free of organic solvents such as are conventionally present in liquid paint systems. Accordingly, economic and social benefits such as reductions in air pollution, energy requirements, and fire and health hazards can be realized through the use of powder coatings.
A common technique for applying a powder coating to an object makes use of electrostatic powder spray coating equipment. In such application, a coating powder is dispersed in an airstream and passed through a high voltage field whereby the coating particles attain an electrostatic charge. These charged particles are attracted to and deposited on the object to be coated which is usually at room temperature. Subsequently, the object is placed in an oven and heated whereby the powder melts/cures to form the desired coating on the object.
U.S. Pat. No. 5,009,367 to Nielsen, U.S. Pat. No. 5,027,742 to Lee et al., and "HIGHER SOLIDS COATINGS ABOVE 80% BY VOLUME," presented at the Water-Borne & Higher Solids Coatings Symposium, Mar. 10-12, 1980, all concern the spraying of materials using supercritical fluids.
Further, based on U.S. Pat. No. 5,158,986 to Cha et al., U.S. Pat. No. 5,334,356 to Baldwin et al., and the article entitled, "NEW ROLES FOR SUPERCRITICAL FLUIDS," appearing in Chemical Engineering, March 1994 (pages 32-35), it is known to feed fluids, including supercritical CO.sub.2, to an extruder to form an extruded shape of a fluid and polymer plastic material. As disclosed, such extruded material can subsequently be processed to form a desired supermicrocellular, foamed material, such as in the form of a sheet.
Conventionally, the manufacture of a powder coating comprises melt-mixing a resin, a curing agent, plasticizers, stabilizers, flow aids, pigments, and extenders. Whereas dry blending is commonly used to make PVC powders under conditions not amenable to the formation of very fine powders, melt-mixing involves the high speed, high intensity mixing of dry ingredients in a Henschel mixer or the like and then the heating of the mixture to an elevated temperature (e.g., about 180-250.degree. F.) in a continuous compounder such as a single or twin screw extruder to achieve thorough dispersion of the other ingredients in the resin as the resin melts, forming a molten mixture. The molten mixture is then cooled to quench the reaction and crushed. Such processing is then generally followed by a sequence of operations which can involve grinding, sifting, separation, and filtering, followed by more sieving.
Such manufacture and processing of coating powders, however, are subject to a number of shortcomings or difficulties. For example, high temperature processing of ingredients in a melt extruder can bring about premature reaction of the resin with the curing agent or degradation of at least some polymer resins.
Additionally, the particles produced as a result of such crushing and grinding operations are generally substantially non-spherical, irregularly shaped. Such irregularly shaped particles can have an undesirable effect on the uniformity and continuity of any resulting coating formed on a substrate surface as a result of application and curing of such a powder coating.
Furthermore, the particles produced by such conventional manufacture processing tend to vary greatly in size. Consequently, various particle separation techniques such as screening and cyclone separation can be required in order to separate undesirable large and small particles from the powder particles having the desired size distribution. The powder particles which are undesirably sized must then typically be downgraded or otherwise disposed of.
In the past, various approaches have been proposed in order to overcome or minimize some of the above-identified problems.
U.S. Pat. No. 5,207,954 to Lewis et al. discloses a method of making a thermosettable, coreactable particulate powdered composition of a first copolymer of an olefinically unsaturated monomer having at least one functional group and at least a second copolymer of an olefinically unsaturated monomer having at least one functional group which is reactive with the functional group of the first copolymer. Aqueous dispersions containing the coreactive polymers are disclosed as being spray dried to produce copolymeric particles which are substantially uniform and spherical in shape.
U.S. Pat. Nos. 4,582,731 and 4,734,451, both to Smith, disclose methods and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in organic and supercritical fluid solvents. The examples disclose the application of single component films to substrate surfaces. These patents do not appear to disclose coating materials composed of multiple components or materials, or the processing thereof.
U.S. Pat. No. 5,290,827 to Shine concerns a process for preparing a homogeneous blend of otherwise thermodynamically immiscible polymers, rather than resins with or without a curing agent. In accordance with the disclosure, mixtures of polymers are dissolved under pressure in supercritical fluid solvents and then expanded through a fine nozzle. As the supercritical fluid solvent evaporates, the polymer mixture is disclosed as depositing as a substantially homogeneous blend.
U.S. Pat. No. 5,399,597 to Mandel et al. discloses a batch process for preparing powder coating materials whereby at least some of the above-identified problems are sought to be minimized or avoided. In accordance with the process thereof, different first and second organic materials and a supercritical fluid are mechanically agitated in a first container. The contents of the fs first container are then discharged into a second container, maintained at a lower pressure than the first container, and in which substantially all of the first and second organic materials are collected.
Such batch processing can suffer from a number of shortcomings. For example, batch processing can undesirably result in long cycle times which, for example, can cause undesired polymerization of fast curing powder coating compositions. Further, batch processing can lead to product inconsistencies, such as inconsistencies in product properties such as viscosity and particle size, due to variations in processing conditions such as pressure and mixing time over the course of a batch run. Still further, large batch runs will typically necessitate the use large processing vessels. Large processing vessels can in turn prove undesirably time consuming to properly clean between runs for or with different product compositions. In addition, in such batch processing it can be difficult to maintain high pressure seals such as typically required to contain supercritical process fluids.
Further, U.S. Pat. No. 5,399,597 emphasizes that with the process disclosed therein, solubilization of components in the supercritical fluid is undesirable as such solubilization would unavoidably result in lose of material upon transfer from the process vessel to the product receiving vessel. The patent teaches the avoidance of such undesirable results through the selection of materials which are not soluble in the supercritical fluid at the operating conditions.